High-voltage transistors, such as a high electron mobility transistor (HEMT), have wide industrial applications in high-voltage and fast switching operations. A gallium nitride (GaN) transistor is a type of HEMT device, which can withstand high drain-to-source voltage (e.g., greater than 100v) and provide fast switching speed. In general, a GaN transistor includes a drain region, a source region, a channel region, and a gate structure positioned above the channel region. For high speed switching, a GaN transistor may also include a field plate above the gate structure. The placement of the field plate may impact the design of the gate structure, which in turn presents several trade-offs between the performance and the size of a GaN transistor.